* {
    margin: 0;
    padding: 0;
}

#box {
    position: relative;
    width: 640px;
    height: 640px;
    margin: 50px auto;
    overflow: hidden;
    border: 1px solid black;
}

li {
    position: absolute;
    font-size: 36px;
    color: #fff;
    line-height: 200px;
    text-align: center;
    width: 200px;
    height: 200px;
    float: left;
    list-style: none;
    font-weight: bold;
    z-index: 0;
}

#box .active {
    background: deepskyblue;
}